What Is a Positive Displacement Pump? Feature, working, Type

A positive displacement pump is a pump type where moving fluid is captured in a cavity and then released a fixed fluid quantity. Fluid displacement occurs through various components, including pistons, plungers, and other parts. These pumps also come with an expanding cavity on the suction side and a reducing cavity on the discharge side.

Fluid is sucked at the inlet side due to cavity expansion and released due to cavity reduction. In this post we will cover feature details for positive displacement pump features and related factors.

What is a positive displacement pump?

• Positive displacement pump passes fluid through repeatedly closing fixed volumes and moving mechanically.
• A positive displacement pump uses changes in volume in the pump cylinder for moving fluid.

•  The pumping process is cyclic and operated with pistons, gears, rollers, screws, vanes, or diaphragms.
• PD pumps also come in rotary or reciprocating positive displacement structures. It has differing inner components and designs for handling different fluid features.
• The basic working features of this pump are as follows.
• The fixed volume of fluid transmitted for one revolution.

Positive Displacement Pump Working

• The positive displacement pump moves fluid in the pump chamber through the inlet valve, and that fluid releases through the outlet valve.
• It comes with a rotary, reciprocating, or diaphragm system that helps fluid to move through the pump.
• It has different parts, like rotary, reciprocating, or diaphragm, that move fluid in the pump. Constant flow rates occur due to repeating cycles. Each pump has its own methods for getting the process.
• The positive displacement pump flow fluid through, holding a certain amount of fluid, and displacing that holding fluid in the discharge pipe
• In displacement pumps there is an expanding cavity on the suction side and a reduced cavity on the discharging part.
• Liquids flow in the pump when the cavity on the suction side expands, and liquid comes out of the discharge when the cavity collapses.
• Volume is constant through every cycle of pump operations. A positive displacement pump does not use impellers but works through a rotating component for pushing liquid in an enclosed cavity till a certain pressure is produced which moves fluid in the discharge system.
• pump not based on increasing fluid when centrifugal pumps do due to moving fluid in the impeller.
• Fluid speed in a positive displacement pump is less as compared to a centrifugal pump.
• It is a required feature for certain uses, like when pumping a medium that has fragile solids.

Types Of Positive Displacement Pumps

The main types of positive displacement pump are as

Reciprocating positive displacement pumps

• The reciprocating positive displacement pump is operated through the back-and-forth motion of a piston, plunger, or diaphragm. These repetitions are called reciprocation.
• In a piston pump, the first piston stroke produces a vacuum, opens the inlet valve, closes the outlet valve, and moves fluid in the piston chamber.
• When piston motion is backward, the inlet valve has pressure and is now closed, and the outlet valve is open, helping fluid in the piston chamber to release.
• The bicycle pump is a common example of this process. Piston pumps operate double through inlet and outlet valves on both sides of the piston.
• If the piston is on the suction side, that means compression on the other side. radial design that is complex and used in industries
• Plunger pumps also work the same; fluid is moved through the piston pump based on cylinder volume. In plunger pumps, it is based on the dimension of the plunger.
•  Seal cover of piston needed for maintaining pumping process for controlling leak.
• The plunger pump seal can be easily maintained because it is static at the upper side of the pump cylinder, and the seal about the piston is moving up and down in the pump chamber.
• The diaphragm pump comes with a more flexible layer than the piston for moving fluid.
• Through expanding the diaphragm, the volume of the pumping chamber is higher, and fluid moves into the pump.
• Diaphragm compression reduces volume, and fluid expulsion occurs. Diaphragm pumps come hermetically sealed, so they are good to use for pumping difficult fluids.
• The cycling process of the reciprocating pump causes pulses in discharge through fluid acceleration at the time of the compression phase and becomes slow at the time of the suction phase.
• It causes vibrations damaging at the time of connection, and some damping is smooth.
• Pulsing is also reduced through the use of 2 or more pistons, plungers, or diaphragms, one used during compression and the other used in suction.

Rotary Positive Displacement Pump

• In pump fluid, move through the rotary component. Due to rotation, fluid moves from storage to the discharge pipe. The common types of these pumps are the internal gear, screw pump, flexible vane, and helical twisted roots.
• the rotary positive displacement pumps that also have 3 main types.

Gear pumps:

• In this pump, fluid flows between 2 rotating gears. Liquid moves between two gears when they rotate.

 Screw pumps:

• This pump comes with 2 screw rotors rotating against each other. When 2 screws move, they suck water from the inlet and transmit it to the outlet.

Rotary vane pumps:

• It works like a scroll compressor that comes with a cylindrical rotor that comes with a vane that is enclosed in a cylindrical shape.
• When it moves on the rotor trape, liquid between the rotor and casing moves through the outlet.

Progressive Cavity Pump

• The progressive cavity pump comes with rotor rotation in an encasing that is a stator. The rotor is metal, and the stator is made with rubber.
• It is like a screw thread; fluid exists between cavities, and the rotary movement of the rotor applies force to the fluid for moving from one side to the other.
• It comes with low- to mid-level capacity, low- to high-pressure, one seal, low shear, and low pulsation.

 Features a positive displacement pump

• There are 2 main types of pumps: positive displacement and centrifugal. A centrifugal pump has high flow and operates with low-viscosity liquids.
• Some chemical plants mostly used centrifugal pumps. but in some applications displacement pumps are used.
• Such pumps as these have high-viscosity fluid-handling features and operate with high pressure and low flow in an effective way.
• It is also good to use for metering features.

 limitations of a positive displacement pump

• Positive displacement pumps are complex in design and have difficult maintenance. It is not producing a high flow rate as compared to centrifugal pumps.
• Positive displacement pumps have low handling of viscosity fluids as compared to centrifugal pumps.
• Rotary pumps use a seal between the rotating component and the pumping hose that produces suction and controls leakage.
• that reduces with low viscosity fluids. It is difficult to avoid slippage from the valve in a reciprocating pump through a low-viscosity feed since high pressure is produced at the time of pumping action.
• Pulsing discharge is also a feature of positive displacement and normally comes in pump designs.
• Pulsation produces noise and vibration in pipes, and cavitation issues cause damage.
• Pulsing is also reduced through different pump cylinders and pulsation dampeners with safe design features.
• But the centrifugal pump generated smooth, constant flow.

Centrifugal vs Positive Displacement pump

 Why Choose a Positive Displacement Pump?

The selection of a positive displacement pump is based on different factors compared to a centrifugal pump.

Viscosity

• Centrifugal pumps come with errors for pumping viscous liquids; they are not good to use, but positive displacement pumps are easy to use for thick liquids.

Low Flow

• centrifugal pump flow before low flow demand according to requirements. Operating a centrifugal pump off its BEF CAUSES high energy use, affects the pump, and affects complete working.
• The positive displacement pump is good to use since it provides a constant flow of fluid at pump speed.
• If getting pressure using a positive displacement pump is good, it is better to use than a centrifugal pump.

Metering process

• Positive displacement pumps are good to use for the metering process. It provides a constant flow that fulfills process requirements. The main type of positive displacement pump is that used for metering. Peristaltic, Gear, Diaphragm Plunger

High Pressure

• Positive displacement pumps are good to use when high pressure is needed, like some types that come with more than 1,000 psi.
• Based on a positive displacement pump, if it works for a closed discharge valve, it produces pressure in the resulting pipe, which can be damaged due to a line burst.

Shear-sensitive fluids

• Centrifugal pumps come with higher speed of operation than positive displacement pumps. High speed causes shear fluids that prefer to use centrifugal pumps, not paste-type fluids.

Applications of positive displacement pumps

• Positive displacement pumps are used for pumping high-viscosity fluids like paints, oil, etc.
• They are used where accurate metering output is needed.
• Contrary to centrifugal pumps, positive displacement output is not affected by pressure. There are different types of positive displacement pumps; each has its own uses.

Piston pump Water

• Its main feature is a reciprocating process through piston seals on O-ring designs. It is used for high-pressure washing, low-viscosity liquids, paints, and oils.

Plunger pump

• It has no seal, self-priming, low-flow, or high-pressure features. so used for the reciprocating process through plunger sealing with packing

Diaphragm pump

• It is used for spraying, water treatment, paints, oils, and dispensing.

Gear pump

• Gears give a rotary pumping process and are used for the chemical, oil, and paint industries and for pumping high-viscosity fluids.

Lobe pump

• It is used for the chemical and food industries and sanitation, and it can be cleaned easily and has low shear.

Screw pump

• It is used for fuel transfer, injection, irrigation, and oil production. In this pump, fluid moves radially and has high flow rates.

Vane pump

• It is used for vehicle transmission systems and fuel loading of low-viscosity fluids.

positive vs non-positive displacement pumps